The present invention relates to a battery pack used as a power source for video cameras, portable telephones or personal computers, a battery charging device for charging the battery pack and a method for charging the battery pack.
Hitherto, there are known battery packs constituted by secondary batteries such as lithium-ion batteries, nickel-cadmium batteries, nickel-hydrogen batteries or the like.
These known battery packs incorporate, for example, a microcomputer for calculating a residual capacity thereof and conducting communications with electronic apparatuses using the battery pack as a power source, peripheral devices, a detection circuit for detecting conditions of a battery cell which are required for the microcomputer to calculate the residual battery capacity, or the like.
In conventional battery charging devices for these battery packs, for example, constituted by lithium-ion batteries, there has been used a constant-voltage charging system in which a predetermined constant voltage is continuously applied when the battery is charged. In addition, the respective battery packs have often been different in charging voltage (which means a maximum charging voltage which can be applied to the battery pack to be charged, and is hereinafter referred to merely as "maximum charging voltage") per one battery cell depending upon kinds of electrodes used in the respective battery cells. For this reason, many of the conventional battery charging devices have been used only for charging limited kinds of battery packs compatible thereto.
However, from the standpoint of costs, it has been inconvenient to prepare many battery charging devices compatible with various types of battery packs. Accordingly, many of recent battery charging devices are obliged to have such a function by which various types of battery packs having different maximum charging voltages can be charged.
However, such battery charging devices capable of charging plural kinds of battery packs have been so designed, from the viewpoint of safety, as to operate in conditions for charging the battery packs having the lowermost maximum charging voltage among those of all the compatible battery packs. Similarly, the battery charging devices have been so designed as to provide such a charging current (which means a maximum charging current which can flow through the battery pack, and is hereinafter referred to merely as "maximum charging current") identical to the lowermost maximum charging current among those of all the compatible battery packs.
Thus, in these conventional battery charging devices, it is impossible to equally supply an optimum charging energy to all the compatible battery packs intended, so that there arise inconveniences that 100% charging cannot be attained or it takes a considerably long time to attain 100% charging.